Fuel coming out of main bowl vent

[HSSS427]

Have a 427 so with a Holley 4150 750 cfm with the ProForm main body on it. I noticed the engine running rough when crusing at about 3000rpm. Got back to the house and ran it with the hood lifted. When I reved it up, fuel started coming out of the main bowl vent on the primary side (the 1/4" tube that sticks straight up between the plenums). I checked the float level and it's where it's supposed to be - a little fuel sloshes out when I rock the car. Jets on the primary side are #73. Any ideas on what would cause this?

I'm thinking more and more about that Demon carb, cause this one's giving me fits......
Thanks!
Scott

[HighPlainsDrifter]

Scott,
You probably have the main needle and seat stuck open with dirt or something. The best thing is to replace it and the fuel filter and ajust the float while you have the bowl off.
Perry.

[trularin]

Perry's right, the needle and seat.

[chuckbrandt]

I had the exact same symptoms, if yours only does this when fully warmed up.

I threw 5 gallons of 100 octane race fuel in the tank and the problem appears to have gone away. (Mixed with half a tank of amoco ultra I bought mid february)

I think it's due to winter gas formulations causing the fuel in the bowl to boil.

If you search for winter fuel on here you'll see some pretty compelling threads to support this.

Like this one:
winter fuel formulation


Chuck

[HSSS427]

I'll pull the needle out and clean it real well, but it is a new one from the re-build a couple of weeks ago. The winter fuel thing sounds kind of weird, but that seems to be the sympton - it does it when it's fully warmed up. I tried again after the car had cooled off for an hour or so and it didn't seem to happen.

I'm still thinking about getting a Mighty Demon 825 cfm. I'm just worried that with this thing hodge-podged from the old Holley and the ProForm stuff that there could be something else wrong. However, if it is the fuel thing, then I guess that would happen with any carb?
Thanks!
Scott

[chuckbrandt]

Needle and seat is an easy enough thing to try, I did too. I bought a trick kit and went through the whole carb and didn't find anything amiss.

My carb is a new holley HP 750 dp so I wasn't inclined to switch carbs because it was an expensive one, but was thinking about the BG at one point too.

Oh, I also lowered the float level slightly, instead of dribbling out it's about 1/8" below, you can see it when you rock the car but it doesn't run out.

But, try the race fuel thing too. You might be surprised. That's an easy thing to try, you'll only be out $25 and can cuss me if it doesn't work.

Chuck

[Rick Parker]

Scott: Sounds like your fuel pressure is excessive & you are overpowering the needle & seat.

Just my 2 cents worth.

Rick

[mj_duell]

My money is on the needle and seat being stuck. I had the same problem with my Mighty Demom last year. Took me week of meddling to figure it out. Real easy to test and fix.



--Mike

[Bruce Edwards]

Do you have a spacer between the carb and intake? The paper thin gasket is going to move to much heat to the carb.
The other thing to do is install the vent whistle. This little jewel will move the vent to the front of the bowl and makes it harder for the fuel to go out the vent.
One other thing you need to check is the full pressure. If it is to high the float will not hold it back and will cause the bowl to over fill.

[vettestr]

You have already got some great advise and thought a link to reg. setting or specs might help. http://www.holley.com/HiOctn/TechSer.../FMFPTech.html
Check the pressure of the pump output for your application. If needle seat are good then must be a high pressure issue. 4 psi street to about 7 psi hot rod is a good to from range but read the link to holley

[chuckbrandt]

I agree he needs to check out fuel pressure and the needle and seat. He needs to cover all the bases.

I called Holley Tech on this issue and that is what they said too. They said I should install a regulator set to no more than 5 1/2" psi. I did buy one but didn't install it because I also bought a new fuel pressure gauge and it reads a real consistent 6 psi from my carter mechanical fuel pump.

If you have an electric pump with no regulator that could be the problem too.

But the fact that (like mine) it only happens when warm is hard to explain by the other issues. Also my problem only started after a particular fuel fill up in mid february.

Anyway, good luck, hope you get it figured out.

Chuck

P.S. Let us know what fixes it, I may have a relapse.

[Bruce Edwards]

Chuck,

Are you using the heat crossover on your intake? What size gasket are you using to mount your carb?
The FE engines will make a lot of under hood heat and will boil the fuel in the carb. It is best to try to keep as much heat of the carb as you can. Shelby used the cold air box to help keep heat of the carb as well as force fresh air in the engine.
Installing a insulator style gasket will help keep the heat away from the carb. Also keeping the fuel lines away from heat will help from pre heating the fuel and getting it close to the boil point. Winter blends are set with a lower temp to help during cold weather so an octane boost in the winter may also help.

[HSSS427]

I'll start with the needle and seat cleaning as that's easiest to do. Then I think I'll order and install a pressure regulator as I've had several people say that including a guy who tuned the carb for me. I do have a 1/2" phenolic spacer under the carb and have a turkey pan as well. And next I'll have to see if I can track down some race fuel, but the only place I've seen it is at Road Atlanta which is a good 2 hour round trip.

I'll post the results as I get them.

As usual, ClubCobra comes through as one of the best collaboration and knowledge sharing sites I know of!
Thanks!
Scott

[HSSS427]

Took the inlet valve / seat assembly out, gave it a good look, blew it out with compressed air, but it looked clean. Reinstalled it and re-set the float level with the car running. Seems to run fine since it's now cooled off. Fuel pressure was reading 7.5. Looks like one point for the winter fuel theory.

I have a mechanical pump and -6AN hard aluminum line that runs straight up from the pump and then curves over under the expansion tank, makes a 90deg right turn by the oil filler tube and connects into a aluminum fuel log that runs parallel to the top edge of the passenger side valve cover. I noticed that the log was in contact with the front valve cover "tab" so I stuck a double layer piece of gasket material in there. I guess there is little in the way of insulation that is keeping the fuel in the line from getting a lot of heat from the engine and valve cover since it's all just aluminum.

I'm considering switching to braided line ie Earl's, Russel, etc. I assume that the rubber inside the line will provide better insulation than the aluminum and I could probably route it further from the block. I'd also probably have to do this to install a pressure regulator anyway as trying to re-shape the tubing will be a pain. Don't know if this will be enough to stop the boiling - if that's the culprit - but appreciate thoughts and feedback on the idea.

Also, would any kind of octane booster or other fuel additive raise the boiling point of the fuel, or do you just have to dilute it down with enough race gas to effectively lower the butane content that's causing the boiling?
Thanks!
Scott

[chuckbrandt]

If you can find a sympathetic person at the airport you might get 100 octane AV gas at the airport.

Good luck, Chuck

[Bruce Edwards]

Any of them will raise the boil point. The whole trick is in making the fuel burn slower. A lower octane will burn faster and under compression will self light, thus the ping sound, the slower burning fuel will not self light as it is not as prone to burn thus it will not cause the ping.
Winter fuel is made to burn at a lower point than summer blends as the cold air does not mix well with the fuel so they make it so it will evaporate faster and mix with the air. Fine when it is 40 out but really bad when it is 90+.

[chuckbrandt]

Hi Bruce,
Regarding carb heat, my Performer RPM intake doesn't have the exhaust cross over. When I put the carb back on after going through it, I did use a different gasket, one that appeared to have some metal in it. I can't really go any thicker because of hood clearance with the air cleaner I'm using, and I don't have a hood scoop so a turkey pan isn't all that practical. I think the air cleaner is probably a big contributor, it's a 14" that wraps down around the carb pretty closely trapping engine heat. I have an H&S to try but had heard they are restrictive even with a K&N filter. I have a hard line from the pump to fuel log also, I wonder if a braided line would keep the fuel any cooler?

Thanks, Chuck

[Bruce Edwards]

Is the gasket you are using a fiber gasket? Should have plastic rings in the corners to stop from over tighting the mounting nuts.
As far as the line you are using it will transfer heat faster than braided line however the factory used steel on the pressure side and it does not cause a problem.

[chuckbrandt]

No it was kind of like an asbestos and tin thing. Sort of like the material in exhaust manifold gaskets. And the real weird thing about it was it had 3 holes with the holes on one side conjoined front to back.

Sorry, hard to explain. I can't find it in the holley catalog or I'd try to post a part number or picture.

To be honest, I doubt the gasket helped much.

I figured the hard line is okay. If it happens again I think my best bet would be to try a smaller air cleaner.

Chuck

[Bruce Edwards]

That gasket is for a ERG cross over in the intake. Go to your local parts store and get a fiber gasket for it They are about 1/4 thick.